The present invention relates to a steam generator or boiler for ensuring the heat exchange between the secondary circuit and the tertiary circuit of a liquid metal-cooled nuclear reactor.
It is known that fast neutron nuclear reactors comprise at least two successive cooling circuits making it possible to pass the maximum amount of heat given off by the fuel elements in the reactor core to turbogenerators, which convert the said heat into electric power. More specifically, the liquid metal, generally constituted by sodium, circulating in the primary circuit transfers the heat given off in the core to heat exchangers, in which part of this heat is transferred to liquid metal, which is also usually formed by sodium, circulating in the secondary circuit. This secondary circuit in turn makes it possible to pass this heat to steam generators, in which it is used for evaporating the water circulating in the tertiary circuit. The thus formed steam drives the turboalternators, which are also located in the tertiary circuit.
The steam generators for ensuring the heat exchange between the primary fluid and the secondary fluid generally comprise a vertically axed cylindrical envelope, in which the sodium of the secondary circuit circulates between the inlet and outlet tubes. Thus, the sodium is in contact with the tubes of a bundle of tubes arranged within the envelope and in which the water of the tertiary circuit circulates.
Such a steam generator is diagrammatically shown in exemplified manner in FIG. 1, in which it is possible to see the cylindrical envelope 10, the inlet tubes 12 for the secondary sodium and preferably located in the vicinity of the upper end of envelope 10 and the outlet tube 14 for the secondary sodium, which in this case issues into the bottom of the generator envelope. Thus, the secondary sodium circulates from top to bottom within envelope 10, which forms the most favourable flow direction, because the formation of steam in the tube 16 of the tube bundle shown in FIG. 1 virtually imposes a bottom to top circulation of the water within these tubes. Tubes 16 pass through the envelope 10 of the steam generator and issue, on the one hand into one or more upstream collectors 18 arranged in the vicinity of the bottom of envelope 10, and on the other hand into one or more downstream collectors 20, positioned slightly below the secondary sodium inlet tubes 12. Within envelope 10, tubes 16 are helically wound around the central core 22, in such a way that they occupy the annular space defined between said core and envelope 10. As is also illustrated in FIG. 1, core 22 is extended up to the secondary sodium outlet tube 14, so that passages 24 must be formed in the lower part of core 22 to permit the flow of secondary sodium from the annular space formed between the said core and envelope 10 up to the outlet tube 14.
It is readily apparent that in a fast neutron nuclear reactor, the steam generators constitute particularly sensitive components, because there is a circulation therein of both liquid sodium and water and the large number of tubes in the bundle necessary for the effectiveness of the heat exchange correspondingly increases the risks of a sodium - water reaction within the generator envelope.
To take account of this risk, it is conventional practice to associate with the steam generator an installation making it possible to rapidly drain the same, when an overpressure due to a sodium - water reaction occurs in the generator. At present, and as is very diagrammatically illustrated in FIG. 1, such an installation comprises an auxiliary drainage tank or reservoir 26 resting on the ground surface 28 and a large diameter pipe 30 connecting the bottom of the steam generator envelope to tank 26. Pipe 30 is normally sealed by at least one burster disk 32, which is sensitive to the pressure prevailing in the generator, so that it bursts when this pressure exceeds a predetermined threshold corresponding to the appearance of a sodium - water reaction in the generator. More specifically, it can be seen in FIG. 1 that the burster disk 32 is located in pipe 30 in the vicinity of the bottom of envelope 10. In existing installations, the drainage tank has a large capacity and is used for storing all the sodium contained in the secondary circuit. This construction of existing installations is relatively unsatisfactory for a number of reasons.
Firstly, differential expansions can occur in pipe 30. Thus, bearing in mind the large cross-section of these pipes indispensable for the rapid draining of the steam generator in the case of a sodium - water reaction, it is relatively uneconomic to compensate these expansions by providing the pipes with expansion bends. Thus, these bends increase the length of the tubes and consequently the costs of the installation. Therefore, and as is illustrated in FIG. 1, the pipes 30 are advantageously equipped with expansion bellows 34. However, it is known that after a certain period of use, the mechanical stability and strength of these bellows tends to decrease. Thus, the presence of such bellows has the effect of introducing a supplementary risk from the safety standpoint.
Moreover, when the burster disk has burst as a result of a sodium - water reaction in the steam generator, it must be replaced before putting the reactor back into operation. This disk is also replaced during periodic maintenance inspections. This operation is difficult in existing installations, as a result of the positioning of the disk in the pipe.
Moreover, although the pipe diameter permits a relatively large sodium flow, the length of said pipe is generally such that the time necessary for the complete emptying of the generator is not as short as would be desired under overpressure conditions produced by a sodium - water reaction.